Hands-On Reactive Programming in Spring 5

Completable

Also, RxJava 2.x has the Completable type that may only trigger the onError or onComplete signals,  but cannot produce the onNext signal. It does not implement the Publisher interface either and has the toFlowable method. Semantically, it corresponds to the Mono<Void> type, which cannot generate onNext signals either.

To summarize, RxJava 2 has more granular semantic distinctions between reactive types. Only the Flowable type is a Reactive Streams complaint. Observable does the same job but without backpressure support. The Maybe<T> type corresponds to Reactor's Mono<T>, and RxJava's Completable corresponds to Reactor's Mono<Void>. The semantics of the Single type cannot be directly represented in terms of Project Reactor as none ...